Field of the Invention
The invention relates to a technology for obtaining information about a target.
Description of the Background Art
Conventionally, on a vehicle control system that follows another preceding vehicle, a vehicle control system that reduces collision with an obstacle or other system, a radar apparatus that obtains information about a target existing in the periphery of a vehicle is used.
The radar apparatus transmits a transmission wave and receives a reflection wave that is reflected by a target such as another vehicle, and then obtains information about the target such as a distance and a relative velocity based on a reception signal. A current radar apparatus receives a reflection wave from a target by a plurality of antennas, and estimates the angle of the target (direction to the vehicle) based on the phases of the reception signals received by the plurality of antennas. As an angle estimation system that estimates an angle of a target, ESPRIT, MUSIC and PRISM are well known.
In the case where there are a plurality of targets at substantially the same distance, the angle estimation system as above estimates the angle of each of the plurality of targets. However, in each of the angle estimation systems, a division possible number that is the number of the targets whose angles are rightly estimated is limited. For example, the division possible number in ESPRIT is “3.” In the case where a radar apparatus receives the reflection waves reflected by the targets that exist at substantially the same distance and whose number exceeds the division possible number, the radar apparatus cannot estimate the angles of the targets rightly, and the reliability of the estimated angles is lowered.
The phenomenon in which the reliability of the angles estimated by a radar apparatus is lowered may be occurred in some peripheral environments where a vehicle is traveling, regardless of the number of actual targets. For example, when a vehicle is traveling in a tunnel, even with one actual target existing, since the reflection waves from the target are reflected by the inner wall of the tunnel, a radar apparatus receives the reflection waves from plural directions. Thus, the radar apparatus is in a state as if the radar apparatus receives the reflection waves from the targets that exist at substantially the same distance and whose number exceeds the division possible number. As a result, the radar apparatus cannot estimate the angles rightly, and the reliability of the angles estimated by the radar apparatus is lowered.
In such a case where the reliability of angles estimated by a radar apparatus is lowered, a vehicle control system may perform erroneous control based on incorrect information about a target.